Brackets are glued on the teeth of the patient to be treated and connected to one another via an orthodontic wire, for the orthodontic treatment of patients having fixed braces. The brackets present a pad for connection with the tooth and a bracket body which receives the archwire.
Orthodontic wires made of a shape memory material are often inserted into the brackets, to start an orthodontic treatment to achieve a “rough” orientation of the teeth. It is only towards the end of the orthodontic treatment that for instance steel wires are used to move the teeth as precisely as possible into a defined position.
The orthodontic wires must have a certain geometry (target geometry) to be able to transmit the desirable strength to the brackets and hence to the teeth. It is known to shape orthodontic wires of steel into the desirable target geometry using pliers.
This does not work with orthodontic wires made of a shape memory material since they do not present the conventional elasticity during deformation.
The main shape memory materials include the Cu—Zn—X (X: Si, Sn, Al) alloys and the intermetallic NiTi alloy (nickel content of approx. 55 weight percent) whereas the NiTi alloy could take on a greater technological significance due to its more favourable properties. The shape memory effect rests on a thermoelastic martensite transformation, on a reversible phase transformation conditioned by shearing the grid planes. The cooling down of the high-temperature phase, called austenite, below the alloy-specific martensite start temperature leads to the phase transformation without form-change and without irreversible plastic deformation, as is the case with steels. Shape memory alloys can be easily deformed in martensitic condition; the reversible deformation can amount to 8% with NiTi. This deformation is durable as long as the alloy is in martensitic condition. Warming-up to a temperature above the alloy-specific austenite start temperature leads then to resetting the original shape.
Document DE 195 40 755 C2 describes the generic manufacture of an archwire made of shape memory material by plastering transfer elements which are inserted in slots of brackets on a model of a target configuration, by loosening the transfer elements from the slots, by replacing sections of the transfer elements with wire reception devices and arranging an archwire into said sections followed by warming-up to achieve durable deformation.
To shape an orthodontic wire made of a shape memory material into a target geometry, said wire is given the desirable target geometry in a special baking mould and then heated up to a specific transition temperature for the shape memory material. The orthodontic wire is subsequently cooled down again and inserted into an orthodontic apparatus of a patient, for instance fixed lingual or buccal brackets, under deformation. The orthodontic wire is again warmed up in the patient's mouth and remembers its target geometry into which the wire then strives to re-deform. The orthodontic wire exerts a force on the brackets during said deformation through which the corresponding teeth are moved.
A shortcoming during the deformation of an orthodontic wire made of a shape memory material to achieve a target geometry lies in that the method is quite wasteful and hence quite costly.
Document DE 698 15 155 T2 describes the calculation of forces which should be exerted by an archwire, in a virtual model.